Ultrastructure and Molecular Phylogenetic Position of Neometanema parovale sp. nov. (Neometanema gen. nov.), a Marine Phagotrophic Euglenid with Skidding Motility

Heteronema is a commonly encountered genus of phagotrophic euglenids that contains very different morphotypes, including elongate gliding species and ovoid skidding forms. We report the first ultrastructural and sequence data from a culture of an ovoid skidding heteronemid, KM051. Cells were 8-23.5 μm long with 22 pellicular strips and a fibrous extracellular layer. The tubular extrusomes had dense centre sections. The feeding apparatus was barely visible by light microscopy, but included two microtubule-supported rods. The flagella had hollow, inflated transition zones, heteromorphic paraxonemal rods, and sheaths of flagellar hairs. The posterior flagellum bore a knob that, unusually, sat >2 μm distal to the flagellar base. No ultrastructural features were uniquely shared by KM051 and the elongate, gliding species Heteronema scaphurum. Conversely, the pellicular microtubule array resembles that in deep-branching primary osmotrophs (Aphagea). 18S ribosomal DNA (18S rDNA) phylogenies showed that KM051 is related to a recently obtained Heteronema c.f. exaratum sequence. These skidding heteronemids are not closely related to H. scaphurum, and instead are closely related to Dinema, Anisonema and specifically, Aphagea. The skidding species in Heteronema are transferred to Neometanema gen. nov. (along with most species of Metanema Klebs, 1893), with KM051 described as Neometanema parovale sp. nov.